Meet hCG!

Pregnancy can be detected by looking for the glycoprotein human chorionic gonadotropin, which functions as a hormone. After an egg has been successfully fertilized, the resulting cell multiplies into a mostly hollow ball of cells called a blastula. Trophoblast cells, the cells on the exterior of the blastula, make and secrete hCG when the blastula implants in the uterus. Then hCG levels continue to increase for the first eight weeks, staying up for the rest of the pregnancy.

Diagram showing hCG secreted by trophoblast cells in the developing blastula. hCG circulates in the bloodstream and enters the urine where it can be detected by a pregnancy test. (*The majority of this figure was created with BioRender.)
hCG is secreted by trophoblast cells in the developing blastula and can be detected in the urine.

hCG is a glycoprotein. Two protein subunits, named alpha and beta, make up about 70% of its mass while sugars make up the remaining 30%. This is what it looks like:

Three-dimensional model of hCG. Two protein subunits with similar ribbon-like structures come together. Some sugars are shown attached to the protein, but not all known sugars are in the image.
Base structure modeled on Protein Imager

The protein parts of hCG are built at ribosomes in the cell’s rough endoplasmic reticulum (ER). Each subunit starts out as a longer protein with extra pieces, pre-alpha and pre-beta. These extra pieces, called signal peptides, tell other parts of the cell where the protein needs to go.

Cartoon hCG with signal peptides highlighted. Each one has a speech bubble saying, "to the endoplasmic reticulum!"
Signal peptides direct the protein where to go.

Building the subunits is only one part of making a protein. A lot of other stuff is going on while the protein is being put together:

  • The signal peptides are removed (after each part gets to its destination, of course)
  • Some sugars are added on
  • Disulfide bridges form to stabilize the protein’s structure
Diagram of hCG's construction. pre-alpha and pre-beta strands are put together in the rough endoplasmic reticulum. There, the two subunits come together, signal peptides are removed, and some sugars are added. Then the partially-completed hCG is transported to the Golgi. In the Golgi, more sugars are added and certain others are removed. The final hCG is secreted and circulated through the body.
hCG is built and modified in several steps within the rough ER and the Golgi.

The two subunits join somewhat like a hug–one disulfide bridge even locks them together!

Doodle of two blob characters hugging overlaid on the three-dimensional model of hCG.
The alpha and beta subunits of hCG join together as in a hug.

After all of this, hCG is transferred to the Golgi to get even more sugars tacked on before it’s ready for secretion.

Since hCG is a hormone, it circulates throughout the body triggering different parts to do something. hCG…

  • regulates the production of other hormones like growth factors and prostaglandins
  • inhibits uterine contractions
  • influences the breakdown of glycogen (stored sugar)
  • somewhat coordinates implantation
Diagram of hCG binding to a receptor on a cell. The following effects are outlined as a result of binding: regulation of growth factors and prostaglandins, decrease in uterine contractions, regulation of glycogen breakdown, and promotion of implantation. (*The membrane and receptor in this figure were created with BioRender.)
Effects of hCG that facilitate pregnancy

Certainly hCG is a critical hormone that synchronizes all kinds of necessary changes at the start of a pregnancy–including letting you know about it!

Wu, et al. Structure of human chorionic gonadotropin at 2.6 A resolution from MAD analysis of the selenomethionyl protein, Structure, Volume 2, Issue 6, Pages 545-558, (Free PDF: (on PDB as 1HCN)

S.F. de Medeiros, R.J. Norman, Human choriogonadotrophin protein core and sugar branches heterogeneity: basic and clinical insights, Human Reproduction Update, Volume 15, Issue 1, January-February 2009, Pages 69–95,

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: